The present invention relates to passive infrared intrusion detectors, and more particularly to an infrared intrusion detector which is arranged for mounting on a ceiling.
In U.S. Pat. No. 4,275,303, which is assigned to the same assignee as the present invention, there is disclosed a passive infrared intrusion detector. The system includes an infrared sensing element within an enclosure and a lens covering an opening of the enclosure. Infrared radiation from an intruder is focused onto the sensing element to cause the system to indicate the presence of an intruder in a protected area. The system also includes a light within the enclosure which can radiate visible light in a direction which indicates the zone of infrared sensitivity.
In intrusion systems of the above type, the detector is generally mounted to a wall of the room or area to be protected so that the sensing element of the detector receives infrared radiation from an intruder on one or more beams of infrared sensitivity extending outward from the detector.
Accordingly, one problem with the prior art detecting devices is that, since they are mounted on a wall, an intruder might have access to the device so that he can tamper with it to prevent actuation of the alarm. Further, the device may be unable to observe an intruder close to the wall. Therefore, it is desirable to place the detector in a location such as the ceiling where it is difficult for an intruder to reach it to affect its operation and from which the entire area can be observed. The optical arrangements of the prior art detectors are such that ceiling mounting of the device does not provide for an adequate field of vision to protect the premises.
Accordingly, it is an object of the present invention to provide a passive infrared detector which is arranged to be mounted on a ceiling.
It is a further object of the invention to provide an infrared detector which has a beam indicating light.